As is widely known, diffractive optical elements that produce diffraction of light can be employed in a variety of applications. For example, wavelength-division multiplexers, optical couplers, optical isolators, and like devices used in optical communications fields can be manufactured employing diffractive optical elements.
Diffractive optical elements generally are manufactured by forming a diffraction-grating layer on a transparent substrate. Diffractive optical elements are grossly divided, based on structural differences in the diffraction-grating layer, into refractive-index-modulated and surface-relief types.
FIG. 19 depicts, in a schematic sectional view, an example of a refractive-index-modulated type of diffractive optical element. This refractive-index-modulated optical element includes a diffraction-grating layer 12a formed on a transparent substrate 11, wherein a refractive-index modulated structure has been created in the diffraction-grating layer 12a. In particular, local regions having a relatively small refractive index n1 and local regions having a relatively large refractive index n2 are periodically formed in alternation in the diffraction-grating layer 12a. This enables the occurrence of a diffraction phenomenon originating in the phase difference that arises between light passing through the regions of low refractive index n1 and light passing through the regions of high refractive index n2.
The diffraction-grating layer 12a having the refractive-index modulated structure can be formed utilizing for example a material whose refractive index is increased by the material undergoing energy-beam irradiation. For instance, increasing the refractive index of Ge-doped quartz glass by means of ultraviolet irradiation is known. Likewise, irradiating quartz glass with X-rays to increase the refractive index of the glass is known. Accordingly, a diffraction-grating layer 12a as illustrated in FIG. 19 can be created by depositing a quartz-glass layer of refractive index n1 onto a transparent substrate 11 and irradiating the glass layer with an energy beam in a periodic pattern to raise the refractive index locally to n2.
FIG. 20 illustrates, in a schematic sectional view, an example of a surface-relief type of diffractive optical element. This surface-relief optical element includes a diffraction-grating layer 12b formed on a transparent substrate 11, wherein a relief structure has been embossed in the diffraction-grating layer 12b. In particular, local regions having a relatively large thickness and local regions having a relatively small thickness are periodically formed in alternation in the diffraction-grating layer 12b. This enables the occurrence of a diffraction phenomenon originating in the phase difference that arises between light passing through the regions of large thickness and light passing through the regions of small thickness.
The diffraction-grating layer 12b having the surface-relief structure can be formed by for example depositing a quartz glass layer onto a transparent substrate 11 and employing photolithography and etching to process the glass layer.
FIG. 21 depicts, in a schematic sectional view, one more example of a refractive-index-modulated type of diffractive optical element. The refractive-index-modulated optical element of FIG. 21 resembles that of FIG. 19, but within a diffraction-grating layer 12c in FIG. 21 local regions having refractive indices n1, n2, n3 in three levels that differ from each other are arrayed periodically. Local regions in this way having refractive indices n1, n2, n3 in three levels can be formed within a diffraction-grating layer 12c by for example depositing onto a substrate 11 a quartz glass layer of refractive index n1 and irradiating the glass layer with an energy beam having two different energy levels.
By means of a diffraction grating that contains local regions whose refractive indices are multi-level, diffraction efficiency can be improved by comparison to the case with a diffraction grating that contains regions whose refractive indices are binary. In turn, as will be presumed from the fact that a diffraction grating that includes multi-level variation in refractive index can have high diffraction efficiency compared with a diffraction grating that contains binary variation in refractive index, a diffraction grating that includes continuous variation in refractive index instead of stepwise variation in refractive index can also have high diffraction efficiency compared with a diffraction grating that contains binary variation in refractive index. “Diffraction efficiency” herein means the ratio of the sum of the diffracted light energies to the energy of the incident light. This means that from the perspective of putting diffracted light to use, greater diffraction efficiency is to be preferred.
FIG. 22 represents, in a schematic sectional view, one more example of a surface-relief type of diffractive optical element. The surface-relief optical element of FIG. 22 resembles that of FIG. 20, but within a diffraction-grating layer 12d in FIG. 22 local regions having thicknesses in three levels that differ from each other are arrayed periodically. Local regions in this way having refractive thicknesses in three levels can be formed within a diffraction-grating layer 12d by for example depositing onto a substrate 11 a quartz glass layer and repeating a photolithographic and etching process on the glass layer two times. Thus by means of a diffraction grating that contains local regions having multi-level thicknesses, diffraction efficiency can be improved by comparison to the case with a diffraction grating that contains simple binary thicknesses.
Although diffractive optical elements of the refractive-index-modulated type described above are manufacturable in principle, in practice producing refractive-index-modulated diffractive optical elements is problematic. The reason is because with the amount of refractive-index variation obtained by irradiating for example quartz glass with an energy beam being at the very most 0.002 or so, creating an effective diffraction-grating layer is difficult.
Consequently, the general practice at present is—as set forth for example in Patent Reference 1, Japanese Unexamined Pat. App. Pub. No. S61-213802, and in Non-Patent Reference 1, Applied Optics, Vol. 41, 2002, pp. 3558-3566—to employ surface-relief types as diffractive optical elements. Nevertheless, the photolithography and etching necessary for fabricating relief diffractive optical elements are considerably complex manufacturing processes requiring a fair amount of time and trouble, besides which controlling the etching depth with precision is no easy matter. What is more, a problem with surface-relief diffractive optical elements is that since microscopic corrugations are formed in the element face, dust and dirt are liable to adhere.